The method and system described herein relates generally to generator service or repair. More specifically, the method and system relates to servicing or repairing a wye ring in a wind turbine generator in-situ.
At least some known wind turbines include machines for converting variable speed mechanical input from blades of the wind turbine into electric power that is compliant with an electrical grid. For example, some known wind turbines include a doubly fed induction generator (DFIG) for converting the variable speed mechanical input.
Some known DFIG generator rotors have a floating neutral point. This is frequently provided by a wye ring. The wye ring is typically made from a copper bar and is located at the non-drive end (NDE) of the generator. Due to operational stresses which fatigue the brazed connection between the wye ring and its rotor connection points (or terminal lugs), cracks can develop which lead to discontinuity. When the first crack occurs, the generator continues to function satisfactorily since the current can still reach all three rotor connection points. However, if a second crack occurs in the wye ring, at least one part (e.g., one phase) of the rotor windings are now disconnected from the floating neutral. This results in severe arcing across one of the cracks, and leads to failure of the insulation around the wye ring. Eventually, cross-over arcing occurs between the wye ring and the phase lead. The wind turbine monitoring system detects this cross-over arcing condition and recognizes it as a phase fault, and accordingly shuts the wind turbine down.
In the past, the only way to repair a cracked wye ring was to replace the entire generator. Shipping a new generator from storage to a remote wind site, and then shipping the faulty generator back to storage is an expensive task that can often times exceed the cost of a new generator. Further, to accomplish this repair, a crane capable of lifting heavy loads (e.g., 10 metric tons) to great heights (e.g., 80 meters-100 meters) is required. Cranes of this type are expensive and the generator replacement operation is costly and time consuming. In addition, the wind turbine must be out of service until the new generator is installed.